Various types of transistors and optical devices are manufactured by using semiconductor materials. These semiconductor devices are generally manufactured as follows: A predetermined semiconductor material is made to epitaxially grow on a substrate made of a predetermined material to form a laminated structure consisting of one or more semiconductor layers, and then, after treatment such as etching, doping, patterning, and the like is performed on the semiconductor laminated structure, electrodes are mounted at predetermined locations.
As a semiconductor material for manufacturing such a semiconductor device, a III-V compound semiconductor represented by GaAs has so far been used widely.
Among III-V compound semiconductors, a III-V compound semiconductor nitride such as BN, AlN, InN, and GaN, especially a GaN compound semiconductor such as GaN, AlGaN, InGaN, and AlInGaN, is anticipated as a material for an optical semiconductor device because it operates in a wide band from visible light to ultraviolet light and also can operate at high temperatures. In particular, among the GaN compound semiconductors, GaN attracts attention as a material for a blue light emitting diode.
When a semiconductor device is manufactured by using the GaN compound semiconductor, a GaN single crystal substrate is needed.
However, in preparing GaN single crystals, the horizontal Bridgman method, the pull method, etc., which have usually been used as a crystal growth technique for a III-V compound semiconductor such as GaAs, cannot be used because GaN, whose crystal structure is of a wurtzite type, has a melting point exceeding 2000.degree. C. and moreover has a high vapor pressure at the melting point. Therefore, bulk crystals of GaN are difficult to manufacture, so that a GaN single crystal wafer cannot be manufactured.
For this reason, in preparing GaN single crystals, the epitaxial growth method must be used.
In this case, since a GaN single crystal wafer cannot be used as a substrate for crystal growth, a material of a different type such as sapphire must be used as a substrate for crystal growth.
However, since the difference in lattice constant between sapphire and a GaN compound semiconductor is 20% or more, even if a GaN compound semiconductor is made to epitaxially grow directly on a substrate made of sapphire, many lattice defects exist in the obtained crystal, so that the quality required for actual use cannot be achieved.
To solve this problem, conventionally, a buffer layer consisting of amorphous AlN is once formed on a sapphire substrate, for example, by the metal-organic chemical vapor deposition method (MOCVD method), and then a single layer or plural layers of GaN, AlGaN, InGaN, etc. are stacked on the buffer layer. Alternatively, after a buffer layer consisting mainly of amorphous GaN is formed beforehand on a sapphire substrate by the MOCVD method in the same way, GaN is made to epitaxially grow to a great thickness on the buffer layer.
In either case, a buffer layer consisting of amorphous AlN or GaN is formed on the surface of, for example, a sapphire substrate, which is a material of a different type, and then the intended GaN compound semiconductor is made to epitaxially grow.
This amorphous buffer layer can be formed at a temperature lower than the temperature at which epitaxial crystals for a component of a buffer layer are made to grow. For example, an amorphous GaN buffer layer can be formed by the MOCVD method at a temperature of 500 to 600.degree. C., or it can be formed by the gas source molecular beam epitaxial method (GSMBE method) at a temperature of 500 to 550.degree. C.
However, the conventional formation of an amorphous buffer layer presents problems described below.
First, there is a problem in that the surface condition of an amorphous buffer layer is difficult to control.
This buffer layer is formed so that an amorphous substance such as AlN and GaN having grown in an island form on the sapphire substrate grows planarly to cover the surface of a sapphire substrate. Therefore, to enhance the smoothness of the formed buffer layer, the film thickness of the buffer layer must be increased up to a thickness not less than a certain value. The increased film thickness of the buffer layer is necessary not only from the viewpoint of the security of the aforesaid surface smoothness, but also from the viewpoint of the decrease in the difference of the lattice constant between the sapphire substrate and the epitaxially growing crystal of the GaN compound semiconductor formed on the buffer layer and the enhancement of crystallizability of the epitaxially growing crystals. For this reason, when the buffer layer is made of amorphous AlN, the film thickness thereof is about 10 to 50 nm.
However, as the film thickness of buffer layer increases, the crystallization state of the buffer layer changes from an amorphous state to a polycrystalline structure, and the surface condition is made rough. Therefore, the crystallizability of the GaN compound semiconductor formed on the buffer layer is deteriorated. Whether the surface condition of the buffer layer has been deteriorated or cannot be known even by using a refractive high energy electron diffraction (RHEED) apparatus in the process of film formation of the buffer layer because the buffer layer is not made up of single crystals.
Thus, for the amorphous buffer layer, it is difficult to control the surface smoothness thereof. Therefore, the crystallizability of the epitaxially growing crystals of the GaN compound semiconductor formed on the buffer layer is sometimes deteriorated. Consequently, the manufactured semiconductor device has a poor property and low reliability.
Also, the increased film thickness of the buffer layer prolongs the time taken for forming the buffer layer. Therefore, the productivity in manufacturing the intended semiconductor device is decreased, and also an expensive sapphire substrate is used, so that the semiconductor device becomes very expensive.
Thereupon, although an LED blue light emitting device can be obtained by using a GaN compound semiconductor, the quality and cost thereof are not necessarily satisfactory. The development of a higher quality and more inexpensive GaN compound semiconductor device is demanded.